Method for producing hexamethylene diamine

ABSTRACT

A process for catalytic hydrogenation of adiponitrile to hexamethylenediamine at elevated temperature and elevated pressure in the presence of catalysts based on elemental iron as catalytically active component and ammonia as solvent comprises 
     a) hydrogenating adiponitrile at from 70 to 220° C. and from 100 to 400 bar in the presence of catalysts based on elemental iron as catalytically active component and ammonia as solvent to obtain a mixture comprising adiponitrile, 
     6-aminocapronitrile, hexamethylenediamine and high boilers until the sum total of the 6-aminocapronitrile concentration and the adiponitrile concentration is within the range from 1 to 50% by weight, based on the ammonia-free hydrogenation mixture, 
     b) removing ammonia from the hydrogenation effluent, 
     c) removing hexamethylenediamine from the remaining mixture, 
     d) separating 6-aminocapronitrile and adiponitrile from high boilers individually or together, and 
     e) returning 6-aminocapronitrile, adiponitrile or mixtures thereof into step a).

BACKGROUND OF THE INVENTION

This invention relates to a process for catalytic hydrogenation ofadiponitrile to hexamethylenediamine at elevated temperature andelevated pressure in the presence of catalysts based on elemental ironas catalytically active component and ammonia as solvent, whichcomprises

a) hydrogenating adiponitrile at from 70 to 220° C. and from 100 to 400bar in the presence of catalysts based on elemental iron ascatalytically active component and ammonia as solvent to obtain amixture comprising adiponitrile,

6-aminocapronitrile, hexamethylenediamine and high boilers until the sumtotal of the 6-aminocapronitrile concentration and the adiponitrileconcentration is within the range from 1 to 50% by weight, based on theammonia-free hydrogenation mixture,

b) removing ammonia from the hydrogenation effluent,

c) removing hexamethylenediamine from the remaining mixture,

d) separating 6-aminocapronitrile and adiponitrile from high boilersindividually or together, and

e) returning 6-aminocapronitrile, adiponitrile or mixtures thereof intostep a).

DESCRIPTION OF RELATED ART

U.S. Pat. No. 3,696,153 discloses hydrogenating adiponitrile tohexamethylenediamine at temperatures of 100 to 200° C. and pressures ofabout 340 atm in the presence of granulated catalysts comprising verypredominantly iron and small amounts of aluminum oxide and in thepresence of ammonia as solvent.

Hexamethylenediamine yields of 98.8%, 98.8%, 97.7% and 97.7% are reachedin the examples of Table 1 (run 2) and Table 2 (runs 1 to 3) atpressures of 340 atm. Complete conversion is reported for the firstthree examples and 99.9% conversion for the fourth example. With regardto the life of the iron catalysts, Tables 1 and 2 merely reveal thatcatalyst activity is high at the end of the runs (after around 80 to 120hours).

U.S. Pat. No. 4,064,172 discloses hydrogenating adiponitrile tohexamethylenediamine at pressures of 20 to 500 bar and temperatures of80 to 200° C. in the presence of iron catalysts synthesized frommagnetite and in the presence of ammonia. A hexamethylenediamine yieldof 98.2% is reported in Example 1.

U.S. Pat. No. 4,282,381 describes the hydrogenation of adiponitrile tohexamethylenediamine with hydrogen at temperatures of 110 to 220° C. anda pressure of about 340 atm in the presence of ammonia and ironcatalysts. The hydrogenation effluent contains 0.04 to 0.09% by weightof adiponitrile and 0.2 to 0.5% by weight of 6-aminocapronitrile.

McKetta, Encyclopedia of Chemical Processing and Design, Marcel DekkerInc. 1987, volume 26, page 230, Table 3, confirms that a typicalhydrogenation product contains 0.01 to 0.11% by weight of adiponitrileand 0.10 to 0.21% by weight of aminocapronitrile. Illustrations 2 and 4reveal that these small aminocapronitrile quantities can be separatedoff and returned into the hydrogenation.

These processes suggest that the reaction conditions in the industrialproduction of hexamethylenediamine have to be directed to achievingcomplete conversion of the adiponitrile and of the 6-aminocapronitrileintermediate of the hydrogenation.

The disadvantage with this is that this requires a relatively hightemperature and a very high reaction pressure. If the adiponitrile and6-aminocapronitrile conversion decreases markedly in the course of thehydrogenation, it has to be pushed back up again by raising thetemperature and optionally the reaction pressure and/or lowering thecatalyst loading, or a not inconsiderable loss of product of value willbe incurred.

If, to obtain complete conversion, the temperature cannot be furtherincreased because of decreasing hexamethylenediamine selectivity and/orthe pressure cannot be further increased for technical reasons, then thecatalyst loading has to be reduced. However, this means that catalystproductivity, i.e., the amount of hexamethylenediamine produced per unittime, will decrease. If the productivity drops below a certain level,the hydrogenation plant has to be shut down and the iron catalyst movedand replaced with an unused or regenerated catalyst. The greater thefrequency of such shutdowns required per year, the lower thehexamethylenediamine quantity which a given production plant can produceper year.

It is an object of the present invention to provide a process for thecatalytic hydrogenation of adiponitrile to hexamethylenediamine in thepresence of catalysts comprising very predominantly elemental iron andammonia as solvent in an economical and technically simple manner whileavoiding the disadvantages mentioned.

DETAILED DESCRIPTION OF THE INVENTION

The process of the invention does not require complete adiponitrile and6-aminocapronitrile conversion. This provides distinctly highercatalysts onstream times at lower pressures, fewer shutdowns for thehydrogenation plant and hence distinctly higher hexamethylenediamineproductivities compared with the prior art.

It was unforeseeable and hence it is surprising that recycling6-aminocapronitrile, adiponitrile or mixtures thereof into thehydrogenation stage does not cause any shortening of the catalystonstream time. It is also surprising that the entire recycle does notcause any troublesome buildup of by-products in the system.

The adiponitrile used in the process of the invention can generally beprepared by conventional processes, preferably by reaction of butadienewith hydrocyanic acid in the presence of catalysts, especially nickel(0) complexes and phosphorus-containing cocatalysts, via pentenenitrileas intermediate.

The catalysts used can be conventional iron catalysts known for theproduction of hexamethylenediamine by hydrogenation of adiponitrile.Preferred catalyst precursors are those which comprise from 90 to 100%by weight, preferably from 92 to 99% by weight, based on the total massof the catalyst precursor, of iron oxides, iron(II, III) oxide, iron(II)oxide, iron(II) hydroxide, iron(III) hydroxide or iron oxyhydroxide suchas FeOOH. It is possible to use synthetic or naturally occurring ironoxides, iron hydroxides or iron oxyhydroxides, magnetite, which has theidealized formula of Fe₃O₄, brown ironstone, which has the idealizedformula of Fe₂O₃ x H₂O, or hematite, which has the idealized formula ofFe₂O₃.

Preferred catalysts are those which comprise

a) iron or a compound based on iron or mixtures thereof,

b) from 0.001 to 5% by weight based on a) of a promoter based on 2, 3,4, 5 or 6 elements selected from the group consisting of aluminum,silicon, zirconium, titanium, vanadium and manganese, and

c) from 0 to 5% by weight based on a) of a compound based on an alkalimetal or on an alkaline earth metal.

Further preferred catalyst precursors are those in which component b)comprises from 0.001 to 5% by weight, preferably from 0.01 to 4% byweight, especially from 0.1 to 3% by weight, of a promoter based on 2,3, 4, 5 or 6 elements selected from the group consisting of aluminum,zirconium, silicon, titanium, manganese and vanadium.

Further preferred catalyst precursors are those in which component c)comprises from 0 to 5% by weight, preferably from 0.1 to 3% by weight,of a compound based on an alkali or alkaline earth metal preferablyselected from the group consisting of lithium, sodium, potassium,rubidium, cesium, magnesium and calcium.

The catalysts can be supported or unsupported catalysts. Examples ofsuitable support materials are porous oxides such as aluminum oxide,silicon oxide, alumosilicates, lanthanum oxide, titanium dioxide,zirconium dioxide, magnesium oxide, zinc oxide and zeolites and alsoactivated carbon or mixtures thereof.

Preparation is generally effected by precipitating precursors ofcomponent a) if desired together with precursors of the promotercomponents b) and if desired with precursors of the trace components c)in the presence or absence of support materials (depending on which typeof catalyst is desired), if desired processing the resulting catalystprecursor into extrudates or tablets, drying and subsequently calcining.Supported catalysts are generally also obtainable by saturating thesupport with a solution of said components a), b) and if desired c), theindividual components being added simultaneously or in succession, or byspraying said components a), if desired b) and c) onto the support in aconventional manner.

Suitable precursors for components a) are generally readilywater-soluble salts of iron such as nitrates, chlorides, acetates,formates and sulfates, preferably nitrates.

Suitable precursors for components b) are generally readilywater-soluble salts or complexes of the aforementioned metals andmetalloids such as nitrates, chlorides, acetates, formates and sulfates,preferably nitrates.

Suitable precursors for components c) are generally readilywater-soluble salts of the aforementioned alkali metals and alkalineearth metals such as hydroxides, carbonates, nitrates, chlorides,acetates, formates and sulfates, preferably hydroxides and carbonates.

Precipitation is generally effected from aqueous solutions, selectivelyby addition of precipitants, by changing the pH or by changing thetemperature.

The catalyst prematerial thus obtained is usually dried, generally atfrom 80 to 150° C., preferably at from 80 to 120° C.

Calcination is customarily effected at temperatures within the rangefrom 150 to 500° C., preferably from 200 to 450° C., in a gas streamcomprising air or nitrogen.

After calcination, the catalyst material obtained is generally activatedby exposing to a reducing atmosphere, for example by exposing it forfrom 2 to 100 hours to a hydrogen atmosphere or to a gas mixturecomprising hydrogen and an inert gas such as nitrogen at from 200 to500° C., preferably at from 250 to 400° C. The catalyst loading duringthis activating step is preferably 200 1 per liter of catalyst.

The activation of iron catalysts by reduction of iron oxides withhydrogen can be carried out in a conventional manner, for example asdescribed in U.S. Pat. No. 3,758,584, with mixtures of hydrogen andammonia at from 300 to 600° C. or, as described in U.S. Pat. No.4,480,051, in three steps, a first step of reducing the iron oxide withhydrogen or mixtures of hydrogen and ammonia, a second step of treatingthe resulting elemental iron with an oxygen-comprising gas, and then athird step of repeating the reduction of the first step.

U.S. Pat. No. 3,986,985 describes a deeper stabilization of reducedpyrophoric iron catalysts, for example in order that they may betransported. The original catalytic activity can be restored by a brieftreatment of the stabilized catalyst with hydrogen.

The activation of the catalyst is advantageously carried out directly inthe synthesis reactor, since this customarily dispenses with theotherwise necessary intermediary step, i.e., the passivation of thesurface, customarily at from 20 to 80° C., preferably at from 25 to 35°C., by means of nitrogen-oxygen mixtures such as air. The activation ofpassivated catalysts is then preferably carried out in the synthesisreactor at from 180 to 500° C., preferably at from 200 to 400° C., in anatmosphere comprising hydrogen.

The catalysts may preferably be used as fixed bed catalysts in upflow ordownflow mode or else as suspension catalysts.

The hydrogenation can be carried out batchwise, but is preferablycarried out continuously using suspended, but preferably fixed bed,catalysts in the presence of ammonia.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagramatic representation of a first embodiment of theprocess.

FIG. 2 is a diagramatic representation of a second embodiment of theprocess.

If fixed bed catalysts are used, the fixed bed reactor R 1 (see FIGS. 1and 2) can be operated in downflow or upflow mode. It is possible inthis connection to employ the operating mode of a straight pass throughone reactor or through a plurality of consecutive reactors with orwithout intermediary cooling or an operating mode involving one or morereactors with product recycling in the liquid circulation system aroundthe reactor(s).

The reaction temperature is generally within the range from 70 to 220°C., especially within the range from 80 to 170° C., and the pressure isgenerally within the range from 100 to 400 bar, especially within therange from 150 to 350 bar, particularly preferably within the range from200 to 250 bar.

The catalyst loading is customarily within the range from 0.1 to 3 kg ofadiponitrile/l of cat. x h, especially within the range from 0.5 to 2 kgof adiponitrilele of cat. x h.

The parameters, such as temperature, pressure and catalyst loading, foradjusting the sum total of the 6-aminocapronitrile concentration and theadiponitrile concentration, based on the ammonia-free hydrogenationmixture, in the reactor effluent to the range from 1 to 50% by weight,preferably 2-40% by weight, particularly preferably 3-40% by weight,especially 5-30% by weight, required by the invention can be easilydetermined by means of a few simple preliminary experiments.

The hydrogenation effluent of step a) has the ammonia removed from it instep b) in a conventional manner, preferably by distillation, forexample as described in DE 19548289. The ammonia can then with advantagebe reused in step a).

The mixture then has removed from it in a conventional manner,preferably by distillation, hexamethylenediamine and the by-producedhexamethyleneimine. In the case of a distillative removal, this can beaccomplished in a plurality, such as two or three, columns or preferablyone column K1.

The hexamethylenediamine obtained in step c) can then be purified in aconventional manner, preferably by distillation.

The product stream remaining after step c) comprises adiponitrile,6-aminocapronitrile, by-products and compounds having a boiling pointabove that of adiponitrile (“high boilers”). These include nitrogenbases, such as 2-(5-cyanopentylamino)tetrahydroazepine and2-(6-aminohexylamino)tetrahydroazepine. In step d), 6-aminocapronitrileand adiponitrile are removed from this product stream in a conventionalmanner, preferably by distillation, individually or together with highboilers. In the case of distillative removal, this can be accomplishedin plural, such as two (K2 a and K2 b in FIG. 2) or three, columns orone column (K2 in FIG. 1). In the case of one column (K2), it isadvantageous to obtain adiponitrile by sidestream takeoff,6-aminocapronitrile overhead and high boilers as bottom product.Adiponitrile can be converted in the presence of the nitrogenous basespresent in the bottom products, such as2-(5-cyanopentylamino)tetrahydroazepine and2-(6-aminohexylamino)tetrahydroazepine, into substantial amounts of1-amino-2-cyanocyclopentene. Pure adiponitrile, in contrast, gives riseonly to small amounts of 1-amino-2-cyclopentene at base of columntemperatures of 200° C.

The 1-amino-2-cyanocyclopentene content, based on adiponitrile, in theadiponitrile used in step a), which comprises fresh adiponitrile and6-aminocapronitrile, adiponitrile or mixtures thereof recycled from stepe), should be below 5000 weight ppm, advantageously within the rangefrom 10 to 5000 weight ppm, preferably within the range from 10 to 3000weight ppm, particularly preferably within the range from 10 to 1500weight ppm, especially within the range from 10 to 100 weight ppm.

Lowering the level of 1-amino-2-cyanocyclopentene content in theadiponitrile used in step a), which comprises fresh adiponitrile and6-aminocapronitrile, adiponitrile or mixtures thereof recycled from stepe), increases the yield of 6-aminocapronitrile and hexamethylenediamineand facilitates the purification of hexamethylenediamine.

In the case of a distillative removal, the base of column temperatureshould be advantageously below 220° C., preferably below 190° C.,especially below 185° C., and because of the low vapor pressure of thecompounds to be separated a base of column temperature of at least 100°C., preferably at least 140° C., especially at least 160° C., isadvisable. The pressures at the base of the column should beadvantageously within the range from 0.1 to 100, especially from 5 to40, mbar. The residence times of the bottom products in the distillationshould advantageously be within the range from 1 to 60, especiallywithin the range from 5 to 15, minutes.

In a preferred embodiment, these distillation conditions are applied tothe removal of adiponitrile from high boilers. In a preferredembodiment, the bottom product contains 1 to 80% by weight ofadiponitrile, based on high boilers. Further adiponitrile maysubsequently be obtained from this product stream, advantageously in anevaporator at a pressure of from 1 to 50 mbar, preferably from 2 to 25mbar.

In step e), 6-aminocapronitrile, adiponitrile or mixtures thereof arereturned into step a).

The present invention likewise proposes that hexamethylenediamine beremoved, together with 6-aminocapronitrile, from the mixture obtained instep b) and then the mixture of hexamethylenediamine and6-aminocapronitrile be separated into the two components.

In a further preferred embodiment, the adiponitrile stream to bereturned into step a) has by-products, especially1-amino-2-cyanocyclopentene, removed from it in a conventional manner,for example by distillation or extraction.

In a further preferred embodiment, the adiponitrile stream to bereturned into step a) is purified in a conventional manner, for exampleby treatment with an inorganic acid, such as mineral acid, organic acid,such as carboxylic acid, or an acidic ion exchanger or by treatment withan oxidizing agent, such as air, ozone, hydrogen peroxide or aninorganic or organic peroxide.

The process of the present invention surprisingly provides distinctadvantages with regard to the hydrogenation, the distillativepurification of hexamethylenediamine and the onstream time of thehydrogenation catalyst.

We claim:
 1. A process for catalytic hydrogenation of adiponitrile tohexamethylenediamine at elevated temperature and elevated pressure inthe presence of a catalytically active elemental iron component andammonia as solvent, which comprises a) hydrogenating adiponitrile atfrom 70 to 220° C. and from 100 to 400 bar in the presence of catalystsbased on elemental iron as catalytically active component and ammonia assolvent to obtain a mixture comprising adiponitrile,6-aminocapronitrile, hexamethylenediamine and high boilers until the sumtotal of the 6-aminocapronitrile concentration and the adiponitrileconcentration is within the range from 1 to 50% by weight, based on theammonia-free hydrogenation mixture, b) removing ammonia from thehydrogenation effluent, c) removing hexamethylenediamine from theremaining mixture, d) separating 6-aminocapronitrile and adiponitrilefrom high boilers individually or together, and e) returning6-aminocapronitrile, adiponitrile or mixtures thereof into step a). 2.The process of claim 1, wherein the separating of the adiponitrile fromhigh boilers is effected distillatively at base of column temperaturesof below 220° C.
 3. The process of claim 2, wherein the separating ofthe adiponitrile from high boilers is effected distillatively at base ofcolumn temperatures of below 185° C.
 4. The process of claim 1, where inthe separating of the adiponitrile from high boilers is effecteddistillatively and the high boilers stream obtained as bottom product isset to an adiponitrile content of from 1 to 80% by weight, based on thehigh boiler content.
 5. The process of claim 4, wherein the mainfraction of the adiponitrile in the stream of high boilers andadiponitrile is removed from the stream in a downstream evaporator atfrom 1 to 50 mbar.
 6. The process of claim 1, which includes reducingthe level of 1-amino-2-cyanocyclopentene by-product in the adiponitrilestream between steps d) and e).
 7. The process of claim 1, furthercomprising treating the adiponitrile stream with an acid between stepsd) and e).
 8. The process of claim 7, wherein the acid used is a mineralacid, a carboxylic acid or an acidic ion exchanger.
 9. The process ofclaim 1, further comprising treating the adiponitrile stream with anoxidizing agent between steps d) and e).
 10. The process of claim 9,wherein the oxidizing agent used is air, ozone, hydrogen peroxide or aninorganic or organic peroxide.
 11. The process of claim 1, wherein the1-amino-2-cyanocyclopentene content of the adiponitrile used in step a)is below 5000 weight ppm based on adiponitrile.